The responsa are thousands of volumes of answers to specific questions on Jewish law. If the Talmud is a law book, then the responsa are case law.
The responsa are composed by well-respected rabbis who have been asked a specific question, and include a full description of the situation, references to the applicable Talmudic passages, the rabbi's answer, and the reasoning behind his opinion. Responsa began to be compiled in the Middle Ages and continue to the present day.
Recent responsa have dealt with topics such as the kashering of dishwashers, cosmetic surgery, and artifical insemination. The Global Jewish Database has compiled 343 books of responsa electronically.